Aerobic oxidation of alcohols over carbon nanotube-supported Ru ...

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132 X. Yang et al. / Applied Catalysis A: General 382 (2010) 131–137phase reaction system. After the reaction, the catalysts can be easilyseparated from the liquid products by sedimentation and can berecycled.90 min, followed by evacuation at 450 ◦ C for 4 h. Before the measurement,the catalysts were evacuated at 40 ◦ C for 2 h.(1)2. Experimental2.1. MaterialsCNTs with a diameter of 10–20 nm, a length of 1–2 m, and N 2surface area of 151 m 2 /g were used. Coconut shell based activecarbon (AC) with N 2 surface area of 986 m 2 /g was purchasedfrom Beijing Broad Activated Carbon Co., Ltd. (Beijing, China). TiO 2(Degussa P 25 )withaN 2 surface area of 59 m 2 /g was purchasedfrom Shanghai Haiyi Scientific & Trading Co., Ltd. (Shanghai, China).RuO 2 was purchased from Alfa Aesar Co., Ltd. Oxygen with a purityof 99.995% was purchased from Dalian Guangming Special TypeGas Co., Ltd. (Dalian, China). All reagents used in this work were ofanalytical grade.2.2. Catalyst preparation2.4. Alcohol oxidationAlcohol oxidation reactions were carried out in a 50 mL threeneckround-bottom flask under magnetic stirring. For a typicalrun, 0.2 g of Ru/CNTs catalyst, 2 mmol of benzyl alcohol, 10 mL oftoluene, and 5 mL of water were placed in a flask that had beenflushed with flowing oxygen or air at atmospheric pressure, andheated at 85 ◦ C for 3 h. The products were analyzed by GC (Agilent6890N) and GC–MS (HP6890–HP5973). After the reaction, theRu/CNTs catalyst was separated from the reaction products by sedimentation,and was subsequently washed first with acetone, thenwith an aqueous solution of NaOH (0.5 M), and finally with water.Thereafter it was dried at 100 ◦ C for 8 h under vacuum before recycling.Before use, pristine CNTs were refluxed in a mixture of HNO 3and H 2 SO 4 (v/v 2:1) at 120 ◦ C for 4 h, then filtered and washed withdeionized water until the pH value of the filtrate reached 7, thendried at 130 ◦ C for 12 h under vacuum. Pristine AC was crushed,washed, and sieved to 200–320 mesh, then treated by the sameprocedure for modifying the CNTs as described above.Ruthenium catalysts supported on CNTs were prepared by awetness impregnation method. The modified CNTs were impregnatedin an aqueous solution of RuCl 3 under ultrasonic conditionsfor 30 min, and then the mixture was incubated at room temperature(RT) for 12 h. The CNT-supported RuCl 3 samples were dried at110 ◦ C for 12 h under vacuum, and reduced at 400 ◦ C in flowing H 2for 2 h, and then cooled to RT in argon, yielding Ru/CNTs catalystswith different Ru loadings. For the Ru/AC and Ru/TiO 2 catalysts, thesame procedure as described above was adopted.2.3. Catalyst characterizationFT-IR spectra of pristine and modified CNTs were recorded ona Jasco-430 spectrometer. The BET surface areas of the modifiedCNTs were measured by N 2 physisorption at −196 ◦ C (MicromeriticsASAP 2020). Ruthenium reducibility was determined bytemperature-programmed reduction (TPR) using a MicrometricsChemisorb 2720 instrument equipped with a thermal conductivitydetector. A non-reduced RuCl 3 /CNTs sample was heated to 750 ◦ Cat 10 K min −1 in a gas stream of 10% hydrogen in argon.The crystalline structure of Ru/CNTs was analyzed by powderX-ray diffraction (XRD, D/MAX-2400) using Cu-K radiationwith a scanning rate of 2 ◦ min −1 . Transmission electron microscopy(TEM) examination was conducted using a Philips Tecnai G 2 20microscope equipped with a CCD camera operated at 200 kV. Thecomposition of the catalyst sample was analyzed by Energy DispersiveX-ray (EDX) Spectrometry.The dispersion of exposed Ru atoms on the supported Ru catalystswas analyzed by H 2 chemisorption using a QuantachromeAutosorb-1 instrument at 40 ◦ C, assuming the chemisorption stoichiometryof H/Ru = 1/1 [24,25]. Before chemisorption, catalystswere heated from room temperature to 450 ◦ Cat4 ◦ Cmin −1 in flowingH 2 (99.999%). The catalysts were reduced in H 2 at 450 ◦ C for3. Results and discussion3.1. Catalyst characterizationThe modification of the CNTs aims to remove amorphous carbonand to increase the number of surface oxygen-containing functionalgroups that are useful for the metal deposition and dispersion.The FT-IR spectra of the CNTs before and after the modificationare shown in Fig. 1, showing that new peaks at 1710 cm −1 and1190 cm −1 appear after modification, which indicates that the carboxylicgroups are formed on the CNTs. The BET surface area of themodified CNTs is 162 m 2 /g.The TPR profile of RuCl 3 /CNTs is shown in Fig. 2. The peak with amaximum at about 155 ◦ C is similar to the reduction temperature ofunsupported RuCl 3 [26,27]. Peaks with maxima at about 248 ◦ C and301 ◦ C are attributed to the reduction of oxygen-containing functionalgroups on the surface. The peak with a maximum at about554 ◦ C is due to the methanation of CNTs. The RuCl 3 /CNTs catalystswere reduced at 400 ◦ C in flowing H 2 for 2 h; under these conditionsRuCl 3 could be completely converted into metallic Ru.Fig. 1. FT-IR spectra of pristine CNTs and modified CNTs.
X. Yang et al. / Applied Catalysis A: General 382 (2010) 131–137 133Fig. 2. TPR curve of RuCl 3/CNTs.Fig. 3. XRD patterns of CNTs and Ru/CNTs.For Ru/CNTs samples with different Ru loadings, no diffractionpeaks related to metallic Ru are detected (Fig. 3), which impliesthat Ru nanoparticles are of nanometer scale. The XRD pattern ofCNTs is remarkably similar to that of graphite, while the XRD patternof activated carbon indicates an amorphous structure (Fig. S1).For CNTs, there exists a symmetrical and sharp 0 0 2 peak at 26 ◦ ,and a broad 1 0 0 peak at 42 ◦ . The average interlayer spacing is calculatedto be 0.34 nm [28,29]. It has been noted that the layeredstructure of CNT is favorable for the transfer of electrons and for aclose interaction between the metal and support by intercalation[30]. Typical TEM images of the 6.0 wt% Ru/CNTs sample, along withits corresponding EDX spectrum, are shown in Fig. 4, which furtherevidences the presence of metallic Ru species. It can be seen fromthe TEM images that the Ru particles with an average size of 2.0 nmare highly dispersed on the surface of CNTs.3.2. Alcohol oxidationThe selective oxidation of benzyl alcohol to benzaldehyde wasused to probe the catalytic activities of the Ru-based catalysts. Theeffect of Ru species, catalyst support, and Ru loading on the catalyticactivity of the supported ruthenium catalyst are shown inTable 1. Among various Ru species tested (entries 1–3), Ru/CNTsshows the highest catalytic activity for the oxidation of benzyl alcohol.For TiO 2 and AC supported Ru catalysts with the same metalloading (entries 4 and 5), the catalytic activity is lower than the CNT-Fig. 4. TEM images and corresponding EDX spectrum of 6.0 wt% Ru/CNTs.
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